1. Field of the Invention
The present invention relates to an imaging apparatus, and more particularly, to a feed roller shaft bearing for an ink jet printer.
2. Description of the Related Art
Traditional inkjet printers operate by incremental motion of media through a print zone. This print zone is typically defined by the width of the ink jet print head. Therefore, for full page coverage, the media can not index more than one print head width. It is critical that on such index increments that the media is moved very precisely, so that there is not a white gap due to overfeeding or a dark band due to overlapping of adjacent print head swaths.
Traditional ink jet indexing is achieved with open loop stepper motor driven systems. These systems typically consist of a stepper motor, gears, and a feed roller. Gear transmission error and variations in the diameter and run out of the feed roller combine to yield an unacceptable level of index accuracy. Therefore, an index which is the full width of the print head is typically done only in a xe2x80x9cdraftxe2x80x9d or xe2x80x9ceconomy-fastxe2x80x9d mode, in which print quality is not critical. In order to produce high quality prints from such a system, a process known as xe2x80x9cshinglingxe2x80x9d is implemented. Shingling involves making multiple passes over a specified area and laying down print swaths one on top of another. While this successfully masks indexing errors, the throughput of the printer suffers a significant penalty.
Another approach is to use a less traditional closed loop indexing system which involves placing a high resolution encoder disc on a feed roller along with an analog output sensor. The use of such a closed-loop system essentially eliminates any errors upstream of the feed roller from the motor and the gears. An essential component of the hardware in a closed loop system for reducing index inaccuracies is the feed roller bearing.
FIG. 1 shows a traditional round bearing design which includes a round bearing 10 which surrounds a round feed roller shaft 12. Generally, a round bearing made of a particular material will exhibit superior wear characteristics in comparison to other bearing configurations made of the same material. However, the tolerances associated with the round bearing design are unacceptable for accurate indexing unless an extremely accurate biasing system is used. Expected tolerances on the traditional round bearing design might be +/xe2x88x920.01 mm on the shaft and +/xe2x88x920.025 mm on the molded bearing, resulting in a 0.07 mm worse case diametral clearance. This clearance could be reduced with costly machine operations, but can not be completely eliminated.
FIG. 2 shows a traditional V-bearing 14 having a first flank 16 and a second flank 18. Flanks 16 and 18 include flat surfaces 20 and 22, respectively, which form a V-shaped cradle for cradling feed roller shaft 12. One advantage of the V-bearing in comparison to the round bearing is that the V-bearing provides for absolute location (0 clearances) between the flat surfaces 20, 22 and feed roller 12. Another advantage that the V-bearing configuration has over the traditional round bearing design is that the bearing does not occupy the area directly above the feed roller and thus, the print head carrier can travel directly over the bearing so as to be as close to the feed roller as possible. The distance between the print head and the feed roller directly impacts the size of the bottom margin of media, and should be held to a minimum. One significant problem associated with the V-bearing design, however, is that high pressure is created at the contact area between the flat surfaces 20, 22 of the flanks 16, 18 and the feed roller 12, resulting in significant wear of V-bearing 14 and/or feed roller shaft 12.
Pressurexc3x97velocity (PV) is a fundamental calculation that is used to select bearing materials for engineering applications. The pressure in round bearings, such as the type shown in FIG. 1, is approximated by the equation: velocityxc3x97[radial load/(bearing diameterxc3x97bearing length)]. The velocity is represented by the surface velocity of the shaft at the bearing interface. The V-bearing design shown in FIG. 2 can not be estimated by the round bearing equation, but can be calculated by estimating the contact area from the deformation of a simple cylinder on a flat plate.
If an application is chosen in which the specified PV limit for a given material is exceeded, a more severe level of wear will result. For example, one such polyimide material having a limiting PV rating of approximately 300,000 PSIxc3x97Ft./Min. is very expensive and difficult to manufacture. Tests conducted on this material in a V-bearing design exhibited a vertical feed roller drop of approximately 0.08 mm over the period of the test. A similar V-design with a more cost effective injection moldable material (and lower limiting PV rating, typically in the range of 10,000-150,000 PSIxc3x97Ft./Min.) would result in significantly increased and unacceptable wear over the same period.
Accordingly, a need exists for an economical bearing which provides absolute location of a feed roller shaft and exhibits low wear characteristics so that the location of the center of the feedroll will not change over time.
One aspect of the invention is a bearing for locating a feed roller shaft in an imaging apparatus. The feed roller shaft includes a shaft surface having a cylindrical shape and has a shaft radius extending from a rotational axis of the feed roller shaft to the shaft surface. The bearing includes a first bearing flank including a first bearing surface having a shape in cross-section defined by a first arc having a first radius extending from a first surface axis. The bearing further includes a second bearing flank including a second bearing surface having a shape in cross-section defined by a second arc having a second radius extending from a second surface axis. The first bearing flank and the second bearing flank are structured and adapted such that the first bearing surface and the second bearing surface together in cross-section form a concave shape, and such that the first bearing surface and the second bearing surface are non-concentric.
In another embodiment, the invention is directed to an imaging apparatus having a frame, a feed roller assembly and a bearing assembly. The feed roller assembly includes a feed roller shaft having a cylindrical shaft surface, and at least one feed roller secured to the feed roller shaft. A shaft radius extends from a rotational axis of the feed roller shaft to the cylindrical shaft surface. The bearing assembly is coupled to the frame for locating the feed roller shaft. The bearing assembly has a plane of symmetry positioned to intersect the rotational axis of the feed roller shaft. The bearing assembly includes a first roller bearing including a first bearing surface having a first radius extending from a first bearing axis, and a second roller bearing including a second bearing surface having a second radius extending from a second bearing axis. The cylindrical shaft surface tangentially contacts each of the first bearing surface and the second bearing surface at an angle in the range of about 21 degrees to about 45 degrees as measured from a line perpendicular to the tangential contact with respect to the plane of symmetry.
An advantage of the present invention is that a feed roller shaft can be precisely positioned in an imaging apparatus for absolute location of the feed roller shaft.
Another advantage is that the invention provides a low-wear design which maintains the feed roller shaft location consistent over the life of the printer, and in particular, essentially eliminates any undesirable vertical drop of the feed roller which would result in an increased printhead to print media gap and change the location of critically positioned components, such as an encoder, which may be mounted to the feed roller shaft.
Still another advantage is that both absolute location of the feed roller shaft and a low wear bearing are provided in a bearing structure using conventional inexpensive thermoplastics, such as injection moldable materials having a limiting PV rating of 10,000 to 150,000 PSIxc3x97Ft./Min.